Uterine contractility is affected by a variety of hormones and other factors. The estrogen/progesterone ratio, the alpha-adrenergic/beta-adrenergic receptor ratio and the prostaglandin content have all been implicated in influencing pregnant and nonpregnant uterine contractility. Substances such as beta-adrenergic agents relax the rat uterus; this action is accomplished by an elevation in cAMP levels, protein kinase activation, and increase in bound cAMP in the tissue. Both cAMP and beta-adrenergic agents increase calcium uptake by microsomal vesicles prepared from uterus. Some actions of beta-adrenergic agents and some other relaxants may not involve the mediation of cAMP. Agents such as prostaglandins and oxytocin, which cause the uterus to contract, antagonize the relaxant-generated cAMP levels under specific conditions. The objective of the present study is to define the molecular mechanisms through which relaxin, a polypeptide with uterine relaxant activity originating in the corpus luteum, affects the uterus. We have previously shown that relaxin can inhibit spontaneous isometric contractions and elevate uterine cAMP, measured by radioimmunoassay. Oxytocin partially antagonized these effect. In the proposed study, tissue from animals in different hormonal states (normal, ovariectomized, pregnant) will be used to define the sensitivity of contractile and cAMP responses to relaxin. Using the most sensitive system, the effect of in vitro treatment of uterine strips with purified relaxin on uterine contractility, response to oxytocin and prostaglandins, cAMP levels, bound cAMP, protein kinase activation ratio, Ca ions uptake and microsomal membrane phosphorylation will be studied. In order to investigate the role that catecholamines may play in relaxin action, beta-agonists and antagonists will be examined for their effects on relaxin action. Catecholamine-depleted tissue will also be used. In vivo studies will be performed to substantiate the in vitro findings and to investigate a possible role for relaxin in promoting the decreased responsiveness of the pregnant uterus.